refactor: move graph related DB code to graph/db from channeldb

This is a pure refactor commit. It moves over all the graph related CRUD
code from `channeldb` to `graph/db`.

channeldb/db.go

@@ -336,7 +336,7 @@ type DB struct {
 	channelStateDB *ChannelStateDB
 
 	dbPath                    string
-	graph                     *ChannelGraph
+	graph                     *graphdb.ChannelGraph
 	clock                     clock.Clock
 	dryRun                    bool
 	keepFailedPaymentAttempts bool
@@ -410,7 +410,7 @@ func CreateWithBackend(backend kvdb.Backend,
 	chanDB.channelStateDB.parent = chanDB
 
 	var err error
-	chanDB.graph, err = NewChannelGraph(
+	chanDB.graph, err = graphdb.NewChannelGraph(
 		backend, opts.RejectCacheSize, opts.ChannelCacheSize,
 		opts.BatchCommitInterval, opts.PreAllocCacheNumNodes,
 		opts.UseGraphCache, opts.NoMigration,
@@ -1370,12 +1370,12 @@ func (d *DB) AddrsForNode(nodePub *btcec.PublicKey) ([]net.Addr,
 		return nil, err
 	}
 	graphNode, err := d.graph.FetchLightningNode(pubKey)
-	if err != nil && err != ErrGraphNodeNotFound {
+	if err != nil && err != graphdb.ErrGraphNodeNotFound {
 		return nil, err
-	} else if err == ErrGraphNodeNotFound {
+	} else if err == graphdb.ErrGraphNodeNotFound {
 		// If the node isn't found, then that's OK, as we still have the
 		// link node data. But any other error needs to be returned.
-		graphNode = &LightningNode{}
+		graphNode = &graphdb.LightningNode{}
 	}
 
 	// Now that we have both sources of addrs for this node, we'll use a
@@ -1647,7 +1647,7 @@ func (d *DB) applyOptionalVersions(cfg OptionalMiragtionConfig) error {
 }
 
 // ChannelGraph returns the current instance of the directed channel graph.
-func (d *DB) ChannelGraph() *ChannelGraph {
+func (d *DB) ChannelGraph() *graphdb.ChannelGraph {
 	return d.graph
 }